My wife doesn't leave our hairdresser, Marina, a tip. But I always manage to somehow slip her a $20 after my wife pays her. No wonder my wife thinks Marina goes out of her way when she cuts our hair—I make sure of it, and I think everyone should. My wife can never figure out why Marina is extra friendly and sometimes stays late for us, but I do.
Probably, a straight rule has not been set yet. But I think it's getting more and more common now. Me personally, I have started leaving a 10 percent tip a while ago.